The present invention relates to certain thermoplastic substrates, such as polycarbonates and polymethylmethacrylates, as well as glazing materials having improved water spreading characteristics. More particularly, the present invention relates to the impregnation of the surface of a thermoplastic substrate with a colloidal inorganic oxide, or by the use of an organic solvent coating composition in the form of a blend of an acrylic resin and a colloidal inorganic oxide whereby improved water wetting and water spreading characteristics are imparted to the treated substrate surface.
Prior to the present invention, as shown by Krautter et al., U.S. Pat. No. 4,576,864, a method for treating plastic material, such as a glazing material to improve its water spreading characteristics was provided by initially treating the plastic substrate with an adhesive layer of a non-water soluble and an essentially non-swellable polymer containing at least one polar group. For example, butyl methacrylate can be mixed with a methyl methacrylate, glycidyl methacrylate and methacrylic acid. After the adhesive layer had dried, there was applied a colloidal inorganic oxide, such as anionic aqueous silicic sol modified with aluminum oxide. The treated substrate was then dried for 5 minutes at temperatures up to about 80.degree. C. Although the method of Krautter et al. provides treated plastic substrates having improved wettability and water spreading characteristics, it requires a multi-step procedure which often is inconvenient convenient if a high production rate is to be maintained during a commercial operation.
An anti-dimming coating for transparent synthetic resin articles is shown in J.P.A. No. 7606193. A mixture of a colloidal silica and alumina in combination with a dispersion or emulsion of a hydrophobic acrylic resin is used to treat the surface of a transparent synthetic resin film. It is reported, however, that adhesion of the coating is completely unsatisfactory, particularly when moistened.
It would be desirable, therefore, to provide thermoplastic substrates having improved wetting characteristics and water spreading characteristics in accordance with methods which would not require multi-step procedures and which would provide coatings on thermoplastic substrates having a satisfactory degree of adhesion.
The present invention is based on the discovery that improved water spreading and wetting characteristics can be imparted to thermoplastic substrates, such as a polycarbonate substrate or a polymethylmethacrylate substrate, if treated with an organic solvent coating composition of an acrylic resin and a colloidal inorganic oxide, such as colloidal silica, as defined hereinafter. Although a heating of the treated substrate until dry is required, no prior application of an adhesive layer is needed. The adhesion of the resulting applied coating to the thermoplastic substrate also has been found to be satisfactory. It also has been discovered that direct impregnation of the surface of the thermoplastic substrate, such as a polycarbonate or polymethylmethacrylate, can be achieved by the use of a mixture of a strong solvent, such as diacetone alcohol, and a colloidal inorganic oxide, such as colloidal silica. The surface of the treated thermoplastic substrate is then dried at elevated temperatures. Although the treated thermoplastic substrates have been found to have enhanced water wetting characteristics and water spreadability, novel properties can also be imparted to the plastic substrate by using different metal oxide colloids, e.g., conduction or antistatic properties with colloidal tin oxide, UV absorbing property with colloidal titanium dioxide, flame retardancy with colloidal antimony oxide and antireflection properties with colloidal silica and alumina.